Downhole packers and bridge plugs have used seals which are compressed by relative movement in conjunction with setting of slips to secure the position of the packer or bridge plug. The slips are cammed outwardly against the tubing or casing to anchor the packer or bridge plug. The relative movement, which is part of the setting procedure for the bridge plug or packer, results in a longitudinal compression of the sealing element or elements, with the compressed position being retained generally through the use of body lock rings. Ultimately, when it is time to release the packer or bridge plug, the locking mechanism of the body lock rings is undermined so that the sealing element can relax, after which the slips are pulled or cammed away from the tubing or casing so that the packer or bridge plug can be retrieved to the surface. One of the problems with sealing elements of the prior designs has been that the seal material, when subjected to extremes of differential pressure, can tend to extrude uphole or downhole, depending on the direction of pressure differential. To counteract this phenomenon, metal back-up rings have been used above and below the sealing element or elements to act as extrusion barriers. The prior technique of mounting such metal back-up rings is illustrated in FIGS. 1 and 2. In FIG. 1a, a typical double-bore packer is illustrated, with the sealing element 10 in set position against the casing 12. The sealing element 10 is made up of several components which are squeezed together in the view shown in FIG. 1a. In between the components is a real separator 14. Above and below the seal assembly 10 are back-up rings 16, which have now been compressed into a generally right angle shape. One surface of the back-up ring 16 bears flush around the periphery of the seal 10 against the casing 12, while the other segment of each of the back-up rings 16 bears against, but is not mechanically connected, to, sleeves 18 or 20. It is the relative movement between sleeves 18 and 20 that compresses the sealing element 10. The back-up ring, for structural rigidity, has a body portion 22 to which the L-shaped segment is connected. by comparing FIG. 1a to FIG. 2a, it can be seen that when it is time to release the packer illustrated, the sleeve 20 has been pulled uphole away from the body portion 22 of the upper metal back-up ring 16. However, the sealing element 10 has failed to relax because the L-shaped portion of the metal back-up ring, in combination with body portion 22, is effectively wedged between the casing 12 and the body 24 of the packer P. Therefore, any further upward force to attempt to retrieve the packer P could stick the packer P in the wellbore, which could require alternate retrieval methods.
This problem occurs because the metal back-up ring structure, which after setting of the element 10 assumes an L-shaped profile in combination with the square cross-section body, is not connected to the moving sleeve 20 such that a release procedure of the packer P does not mechanically separate the back-up ring 16 from the element 10. This lack of connection between the back-up ring 16 and the sleeves 18, 20 can also create problems on insertion of the packer P into the wellbore. Those skilled in the art appreciate that the body elements which ultimately move relatively to each other to compress the seal 10 are held together for run-in. However, with the body 22 not connected to sleeve 18, downward movement of the packer P toward its ultimate setting point can create an upward or compressing force on the sealing element 10 to cause it to expand prematurely before the desired depth is reached. This can cause not only tearing and/or destruction of the sealing element before the packer P reaches its desired depth, but can also result in sticking the packer at a location before it reaches its target depth. Alternatively, sufficient damage to the sealing element 10 Upon insertion of the packer could result in a situation where, when the packer P is set, it will not hold a seal.
The object of the present invention is to provide a seal assembly which will overcome the problem illustrate din FIG. 2a. The objective is accomplished by physically linking a back-up ring to a movable sleeve so that at least one of the back-up rings, on relaxation of the packer, move away from the sealing element 10 to allow it room to relax. Once the back-up ring is moved away from the sealing element in a compressed state, upward movement of the packer will simply collapse the back-up ring since there is no sealing element underneath it. An additional object of the invention is to protect the sealing assembly during run-in to prevent inadvertent expansion which could not only damage the sealing element but also cause the packer to stick in the wellbore at an undesired location. Those and other objects of the invention will be readily understood by a review of the detailed description of the preferred embodiment which is below.